Underwater communications

ABSTRACT

An environmental monitoring system including at least one underwater measurement device and a transmitter for transmitting data from the measurement device to an above water station using a magnetically coupled antenna.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 12/373,010 filedJan. 8, 2009, which is a 371 of PCT/GB07/02937 filed Aug. 2, 2007, whichis the PCT of GB 0615435.5 filed Aug. 3, 2006, all of which applicationsare fully incorporated herein by reference.

DESCRIPTION

The present invention relates to the use of electromagnetic or magneticsignals to provide telemetry data links with environmental and similarmeasuring equipment situated underwater.

BACKGROUND

Increased concern about the environment, especially the quality andcondition of water in rivers, lakes and canals, has raised a generalrequirement for monitoring and measuring parameters relating to water toa much greater extent than hitherto. Due to practical limitations,sampling and testing of water has generally been done manually at only afew locations and with limited frequency. Part of the difficulty is thattesting sites are often at inconvenient and remote places. Whileyielding only inadequate amounts of information, this process is veryexpensive in time, travel costs and labour, and cannot economically beextended to achieve the very much larger number of monitoring points andsampling frequencies required for enabling quick remedial action tocounter pollution sources.

Automatic monitoring and measuring devices have been deployed recentlyin rivers, lakes and canals to increase the frequency of sampling.Commonly called sondes, these sensors measure a large number of possibledifferent parameters relating to the condition of the water.Measurements typically include but are not limited to aspects such as:flow rate; water depth by pressure sensing; water depth by ultrasonictechniques; temperature; acidity/alkalinity (pH); conductivity;dissolved oxygen content; chlorophyll density; salinity and presence ofchemicals such as ammonium salts and nitrates; presence of otherspecific pollutants determined by chemical analysis sensors; presence ofblue-green and other potentially toxic algae; particulate content, andturbidity.

Typically, automatic sondes are raised manually from a river or lakebedby personnel on a boat and their data is downloaded to a data-storagedevice before being transported physically back to a laboratory or datacentre. Usually, the position of each sonde is marked, for example,using a buoy or other marker. Sometimes, the sonde is physicallyconnected to the buoy. In this case, the buoy can be arranged to containa repeater station for taking data from the underwater sonde andtransmitting it onwards by conventional short-range radio to a landstation. Unfortunately, the presence of a buoy is undesirable because itcan draw unwanted attention, with high probability of damage and theftof the equipment. Moreover, in places of natural beauty a visible buoymay be considered unacceptable. A buoy also may preclude the positioningof a sonde in places where there is passing water traffic, such as themiddle of a canal which might otherwise be an optimal location.

In a few cases, sondes are connected by cable laid along the river orlake bottom to a node in an equipment cabinet on dry land near the bank,whence conventional communication methods are used to transmit resultsto a data centre in real-time. While accomplishing the goal of frequentresults nearly in real-time, such a direct cable connection is not anideal solution to the local communication problem. A cable laid alongthe bottom is expensive to install in a manner that will avoidaccidental damage by watercourse users, maintenance operations andfloods, and remain hidden for security reasons. It will usually requireplanning permission and is likely to require a trench in both itsunderwater and on-shore sections. Once a cable is deployed,repositioning of the sonde then becomes prohibitively difficult.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided anenvironmental monitoring system including at least one underwatermeasurement device and a transmitter for transmitting data from themeasurement device to an above water station using a magneticallycoupled antenna. Preferably, transmitter is operable to transmit adigitally modulated electromagnetic or magnetic signal. The sonde andthe transmitter can be separate, but connected units, althoughpreferably, they are included in an integral unit.

Using electromagnetic or magnetic signals as the communication meansavoids the need for a cable connection between a sonde and a cooperativecommunication node or station on-shore. In addition, it avoids the needfor special preparation or installation work. Instead, simplepositioning of the submerged sonde and its transmitting antenna at thebottom of the watercourse is usually all that is required. A furtheradvantage is that by using a magnetically coupled antenna, lowertransmission loss is gained over conventional electromagnetic antennasof the types commonly used in free space. This is because thethrough-water path is in a medium of significant conductivity that,while immediately attenuating an electrical field, leaves a magneticfield largely unaffected.

Environmental monitoring generally requires the transmission of onlyinfrequent small volumes of data and lends itself ideally to the use ofelectromagnetic or magnetic communication, even although the path ispartially through water. A sonde is typically required to report itsresults only occasionally, perhaps once a day but not usually more oftenthan intervals of 15 minutes, a frequency considered almost real-time inthe industry. On each transmission the volume of information is small,often not exceeding a few hundred bits of data. Consequently, forcommunication from/to a sonde, low signal frequencies and bandwidths areeffective. For example, the frequency may be in the range of 100 Hz to100 kHz. For greatest distance, low unwanted signal levels andelectromagnetic noise, a signal frequency in the region of 3 to 5 kHz,preferably 4 kHz, will often be close to optimal. For example, in a 4kHz implementation a sensor submerged in 10 m of fresh water couldcommunicate at 1 kbps with a shore-based station 100 m away.

Conventionally, electromagnetic transmission has not been considered aneffective method when communication is partially or wholly underwater,but our pending patent application PCT/GB2006/002123, the contents ofwhich are incorporated herein by reference, describes how this may beaccomplished over useful distances. One potential difficulty to beconsidered in communicating data underwater is the higher attenuationencountered by an electromagnetic signal when transmitted through thispartially conductive medium. In the environmental monitoringapplications envisaged, the communication distances required arerelatively short, because of the inherent geographical dimensions ofmost bodies of water, and the fact that water is comparatively shallow.Because of this, and especially in the case of fresh water, which hasonly modest conductivity, the necessary distances are generally verysuitable for electromagnetic or magnetic communication. The method andsystem of this invention provide a novel and particularly effectivemeans of communication at low data rates between submerged watermonitoring sondes and a shore station.

Preferably each sonde includes one or more sensors. The sensor(s) may beoperable to measure one or more of the following: flow rate; water depthby pressure sensing; water depth by ultrasonic techniques; temperature;acidity/alkalinity (pH); conductivity; dissolved oxygen content;chlorophyll density; salinity and presence of chemicals such as ammoniumsalts and nitrates; presence of other specific pollutants determined bychemical analysis sensors; presence of blue-green and other potentiallytoxic algae; particulate content; and turbidity.

The invention is equally applicable to fresh water or seawater.Deployment is envisaged in rivers, estuaries, lakes and sea.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention will now be described by way of exampleonly and with reference to the accompanying drawings, of which:

FIG. 1 is a schematic diagram of a communication system between anunderwater sonde and a cooperative station on land nearby;

FIG. 2 a represents a loop or coil antenna;

FIG. 2 b represents another loop antenna in which a core of highmagnetic permeability material is introduced into the coil to form asolenoid of more compact form;

FIG. 3 represents an omni-direction antenna arrangement;

FIG. 4 is a functional block diagram of an underwater modem forcommunication with a sonde device;

FIG. 5 is a functional block diagram of shore based communicationstation, and

FIG. 6 is a schematic diagram of a communication system between anunderwater sonde and a cooperative station on land nearby wherecommunications range is extended by relaying data through anintermediate submerged modem.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an environmental monitoring system thathas a data communication link that is partially or wholly underwater,and is used to convey data gathered by an underwater sonde to acooperative local station, typically on land nearby. When required, thecommunication link may be used to control the operation of the sondeand/or its associated transmitter and receiver. The underwatertransmitter and receiver may be considered a part of the sonde or anadjunct connected to it but, for best operational features, must workeffectively as a single unit. Because the transmission path from thesonde includes an underwater portion, the communication method isradically different from that conventionally applicable to airpropagation systems such as radio. This is primarily because water,especially saline water, exhibits much higher attenuation of the signalover distance. To alleviate the problem of high attenuation, magneticcoupled antennas are used.

Magnetic antennas, formed by a wire loop, coil or similar arrangement,create both magnetic and electromagnetic fields. Close to a transmitantenna there is a predominantly magnetic field that transitions, overan area conventionally known as the near field, to an electromagneticfield with an intrinsic impedance relationship between E and H (electricand magnetic) components characteristic of the medium. The magnetic ormagneto-inductive field is generally considered to comprise twocomponents of different magnitude that, along with other factors,attenuate with distance (r) at rates including a factor proportional to1/r.sup.2 and 1/r.sup.3 respectively. Together these form thepredominant near field components. The electromagnetic field has a stilldifferent magnitude and, along with other factors, attenuates withdistance at a rate including a factor proportional to 1/r. It is oftentermed the far field or propagating component. The far field is the areain which the electromagnetic transmission signal has transitioned to thecharacteristic impedance relationship between E and H components. Thenear field dominates at short distances, whereas the far field isrelatively stronger at greater distances. Dependent on distance betweenthe transmit antenna and the receive antenna, either or both near andfar field components may be used. Using a magnetic coupled antennarather than an electrically coupled antenna reduces signal dissipationin the near field, but allows data transmission in the near and farfields.

In the underwater environment, electrically insulated magneticallycoupled antennas provide various advantages over the alternative ofelectrically coupled antennas. In far field electromagnetic propagation,the relationship between the electric and magnetic field is determinedby the characteristic or intrinsic impedance of the transmission medium.An electrically coupled antenna launches a predominantly electric fieldthat transitions to the characteristic impedance over the near field.Underwater attenuation is largely due to the effect of conduction on theelectric field. Since electrically coupled antennas produce a higherE-field component in the near field the radiated signal experienceshigher attenuation. The same performance issues apply to a receiveantenna. Magnetically coupled antennas do not suffer from these problemsand so are more efficient underwater than electrically coupled antennas.

FIG. 1 shows one embodiment of the invention in a watercourse such as alake, river or canal. This has a sonde 1 that encapsulates transmitterand receiver circuits within its housing, as well as its measurementfacilities. Optionally, the functions may be in separate but connectedunits. Connected to the transmitter/receiver in the sonde is a loopantenna 2. The antenna may be situated on the lakebed or at any pointbetween the surface and lakebed. In the case where data is sent from thesonde, the signal is transmitted 3 to a nearby station, which may be onthe shore, and has a receiver that has an antenna 4 that is connected toa communication module 5. Signals sent from the sonde are received bythe antenna 4 and processed by the communication module 5. Communicationmodule 5 may be housed in a small cabinet as depicted, but could beunderground for reasons of aesthetics or avoidance of unwanted attentionand damage. The antenna 4 could be positioned on the ground,underground, or in other convenient position. It could also bepositioned in the water, perhaps close to the bank. The module 5 maygather and store the data. Alternatively, it may be configured toforward the data by conventional wireline or radio data transmissionmethods to a central monitoring site (not shown).

The shape of the coil antennas 2 and 4 at both ends of the link is notcritical but the received signal will be maximised if they are nearlycircular and each arranged to have a large area, because this createsthe greatest magnetic signal flux for transmitting and interceptsmaximum flux for receiving, both of which increase the received signaland/or enable performance over longer distance. For example, a coilantenna may have a typical cross sectional area of 0.2 square metres butmay be increased or decreased dependent on distance and signal strengthrequired. Multiple turns of one or both coils are usually beneficialbecause this also increases the signal flux created by a transmitantenna and the voltage induced in a receive antenna.

FIG. 2 a shows a simple coil antenna suitable for communication. Thishas multiple turns of wire 6, typically 100, insulated from the waterand for connecting directly to the transmitter and/or receiver so thatsignal current flows in it. FIG. 2 b depicts a similar coil modified tobe more compact but with comparable performance. The coil 8 usual takesa solenoid shape, and has a highly permeable core 7, typically of knownferrite material, introduced within it.

The permeable core multiplies the magnetic flux created by a coil byaround 50 to 100 times, and therefore increases its effective size andability to create and intercept a magnetic field. Another possibleantenna arrangement is shown in FIG. 3. This has three orthogonalsolenoids, which produce in approximation to omni-direction operationfor use in situations where antenna alignment is difficult to maintain.In this case, each antenna is aligned with one of the three Cartesianaxes.

Other aspects of the means of transmitting the data will be familiar tothose skilled in the art, and are given here only in outline. Thesignal, modulated with data from the sonde, can be similar in conceptand form to those found in many conventional radio systems but with amuch lower carrier signal frequency, typically in the region of 4 kHz.The current in a transmitting loop antenna is typically a carrier signalmodulated with the data by one of the many systems well known.Filtering, detection and demodulation of the received voltage induced ina receiving loop antenna again may be by well-known methods.

FIG. 4 is a functional block diagram of a specific implementation of theunderwater sonde 1 of FIG. 1. This has one or more underwater sensors 11for measuring environmental data and a transmitter to allow data to betransmitted to the remote station. Measured data is passed to aprocessor 12. The processor 12 encodes the data to create a bit stream,which is passed to a modulator 13. The modulator 13 synthesises adigital representation of a modulated waveform, which is passed to aDigital to Analogue Converter (DAC) 14. The DAC 14 generates an analoguemodulated waveform which is amplified by a transmit driver 15 and passedto a transmit loop antenna 16 for transmission. Incoming signals fromthe shore station are received at a receive antenna 17 and amplified bya receive amplifier 18. An Analogue to Digital Converter 19 creates adigital representation of the signal and a demodulator 20 extracts adigital data stream. This data is interpreted by the processor 12, whichexecutes the appropriate control function. The equipment is housed in asingle waterproof housing 10 and powered from a battery 22 conditionedby a power supply regulator 23.

FIG. 5 is a functional block diagram of a specific implementation of theshore based communication station 5 of FIG. 1. The diagram illustratesthe similarity with the submerged station for the example of half duplexcommunications provision. Configuration commands are passed from anexternal data interface 31 to a processor 32. The processor 32 encodesthe data to create a bit stream, which is passed to a modulator 33. Themodulator 33 synthesizes a digital representation of a modulatedwaveform, which is passed to the Digital to Analogue Converter (DAC) 34.The DAC 34 generates an analogue modulated waveform which is amplifiedby a transmit driver 35 and passed to a transmit loop antenna 36.Incoming signals from the submerged station are received at a receiveantenna 37 and amplified by a receive amplifier 38. An Analogue toDigital Converter 39 creates a digital representation of the signal anda demodulator 403 extracts a digital data stream. This data isinterpreted by the processor 32, which presents the received data at theexternal data interface 31. The equipment is housed in a housing 30 andpowered from a battery 42 conditioned by the power supply regulator 43.

Operational behaviour of the sonde can be altered by means of commandssent to it directly or indirectly from a central control site or otherorigin. As will be readily recognised, almost any feature of the sondecould be manipulated, but only some examples are given here. Theinterval between transmissions may be altered as desired or eventsrequire: for example the interval could be shortened when a knownenvironmental problem needs special monitoring conditions of greaterfrequency than usual, and decreased on a later occasion. The thresholdlevel at which a parameter measured by the sonde becomes designated analarm condition could be altered from a control site. Selective featuresof the sonde could be commanded to be turned on and off as required. Thepower level of the transmitter could be changed to suit actualconditions of attenuation and distance, possibly as determined by signalstrength received and measured by the cooperative shore receiver,thereby conserving battery energy by using only the minimum necessarytransmitter power for adequate received signal. The sonde could becommanded to report the condition of its battery, thereby avoidingunnecessary early replacements or unexpected expiry failures so thatcostly maintenance visits are optimised. A sonde could be commanded toperform self-testing routines to assess its own operational readinessand accuracy, and to report the results.

It is usually desirable that a sonde located underwater should requirelittle attention and operate from integral batteries for extendedperiods. To conserve battery energy, unnecessary operation or excessivepower of the transmitter should be avoided. To achieve this,intermittent reporting of results can be used. For example, the sondemay be designed to transmit short bursts of data at periodic intervals,perhaps spaced by a known period such as 24 hours, 2 hours or 15minutes, or according to a known schedule, dependent on requirements.This process, which may require less than a few seconds for each brieftransmission, allows the transmitter to consume little or no power forthe vast proportion of time. It should be noted that a knowntransmission interval will also allow a distant receiver to be operableonly at times when a transmission is expected, and so can similarlyconserve energy associated with a receiver at the nearby cooperativestation if desired. As a further measure to conserve battery energy, thesonde receiver can be designed to adopt intermittent operation. If thesystem is arranged to have a predetermined cycle or pattern of possibletimes at which commands may be sent to the sonde, known mutually to bothends of the link, then the receiver in the sonde can be turned offduring the periods in between, thereby avoiding unnecessary batteryconsumption associated with operation of the receiver.

In another enhancement, the sonde may be operable to transmit only whenit has important or urgent data to send, such as on occasions of alarmwhen a key parameter such as a pollution level has been exceeded. Thisavoids both superfluous data and unnecessary battery consumption. Aspreviously outlined, a sonde can be designed to receive and act oncontrol signals sent to it from the land station, but usuallyoriginating from a central control site. Using this capability, afurther enhancement provides for the sonde to transmit data only whencommanded occasionally to do so, thereby avoiding unnecessary energyconsumption by the sonde transmitter.

A further feature of this invention relates to finding a sonde, whichhas been hidden in a watercourse for reasons of security and has no buoyor other marker to indicate its position. A probe receiver, typicallyequipped with a coil antenna, can be used to detect signals from thesonde and home in on its position by means of signal strength, whichincreases with proximity. Electromagnetic signal strength decreasesrapidly with distance in the near field region of a magnetically coupledantenna. Attenuation is increased still further as the signal passesthrough water. A boat equipped with a receiver will measure maximumreceived signal strength when directly above a submerged transmitter.This provides a simple means for location of a submerged transmitter.The high attenuation through water makes this method particularlyeffective since rapid variation of received power with distance allowsaccurate location. This location method is described in more detail inour pending patent application PCT/GB2006/002111, the contents of whichare incorporated herein by reference. In contrast, conventional farfield radio transmission through air experiences very low attenuationand signal strength location methods are rarely used. In a similar way amobile receiver moving along a riverbank or lakeshore will measuremaximum received signal strength when at the closest point to thetransmitter.

FIG. 6 is a schematic diagram of a communication system between anunderwater sonde and a cooperative station on land nearby wherecommunications range is extended by relaying data through anintermediate submerged modem 54. Although a single intermediate relaystation is shown the concept could readily be extended to any number ofrelay devices. FIG. 6 shows a cross-section of a watercourse such as alake, river or canal, and a data communication system to allow anunderwater sonde to communicate with a shore based station through arelay station. Housing 51 encapsulates both sonde and radio transceiverand is connected to antenna 52, which transmits a signal 53 to thereceive antenna 55 of an intermediate transceiver 54. Antenna 55generates a transmit signal 56 which is received by antenna 57 andpassed to surface station 58. The intermediate station 54 can performeither a purely relaying function or can contain a sonde measuringdevice. Where intermediate relay stations contain a sonde device theassociated modem will be individually accessible through an addressingscheme to allow recovery of measured data from each node in the chainthrough the relaying network.

A skilled person will appreciate that variations of the disclosedarrangements are possible without departing from the essence of theinvention. For example, while the foregoing application examples relateto the transmission of data and control signals from/to an environmentalmeasurement sonde underwater, it is readily apparent that the inventionhas greater generality and can be applied to many other systemsrequiring conveyance of measurements or other data from/to underwatersites. In addition, whilst the systems shown in FIGS. 4 and 5 have twoantennas, one for the transmitter and one for the receiver, the sameantenna may be used by transmitter and receiver, but, in the simplestarrangement, usually operative at different times. Accordingly, theabove descriptions of specific embodiments are made by way of examplesonly and not for the purposes of limitation. It will be clear to theskilled person that minor modifications may be made without significantchanges to the operation and features described.

1. An environmental monitoring system comprising at least one underwaterenvironmental measurement device and a transmitter for transmitting datafrom the measurement device in water to a receiver at a control siteusing low frequency electromagnetic and/or magnetic signals, and/or areceiver for receiving data and/or commands from a transmitter at thecontrol site using low frequency electromagnetic and/or magneticsignals, wherein the transmitter and receiver include a magneticallycoupled antenna the system further comprising a processor operable toassess parameters measured by the measuring device in relation topredetermined threshold levels.
 2. A system as claimed in claim 1wherein said predetermined threshold levels are adjustable upon receiptof commands transmitted from the control site.
 3. A system as claimed inclaim 1 wherein said predetermined threshold levels related to an alarmcondition.
 4. A system as claimed in claim 1 wherein said system isoperable to action turning on and/or off of selective features of thesystem.
 5. A system as claimed in claim 1 wherein measured data ismodulated on the electromagnetic or magnetic signal.
 6. A system asclaimed in claim 1, wherein the submerged transmitter is operableintermittently according to a predetermined pattern of transmissionperiods or when instructed to be so operable by one or more commandsignals received by the underwater receiver from the control site.
 7. Asystem as claimed in claim 1, wherein the submerged receiver is operablesubstantially only during periods when a command signal might beexpected periodically or according to a predetermined schedule.
 8. Asystem as claimed in claim 1, wherein the transmitter is caused totransmit data as and when it becomes available.
 9. A system as claimedin claim 1, wherein the underwater device is operative to report thecondition of its battery supply periodically, or on a received command,or when the battery reaches a certain state of discharge.
 10. A systemas claimed in claim 1, wherein the underwater device is capable ofperforming a self-testing or assessment process on receipt of a commandsignal and transmitting the result of the process to the control site.11. A system as claimed in claim 1, wherein one or more of the antennasis omnidirectional.
 12. A system as claimed in claim 11 wherein theomni-directional antenna comprises three orthogonal antennas.
 13. Asystem as claimed in claim 1 further comprising a waterproof housing,wherein the measurement device and transmitter/receiver circuitry areincluded in the housing.
 14. A system as claimed in claim 1 wherein thetransmitter is operable to transmit a signal to allow its underwaterlocation to be determined.
 15. A system as claimed in claim 14 whereinthe position location signal is a regular operational signal or a signalsent as a result of a command for the purpose or a signal sent inresponse to one or more predetermined conditions detected by themeasurement device.
 16. A system as claimed in claim 1 wherein themeasurement device is operable to measure one or more of the following:flow rate; water depth by pressure sensing; water depth by ultrasonictechniques; temperature; acidity/alkalinity (pH); conductivity;dissolved oxygen content; chlorophyll density; salinity and presence ofchemicals such as ammonium salts and nitrates; presence of otherspecific pollutants determined by chemical analysis sensors; presence ofblue-green and other potentially toxic algae; particulate content; andturbidity.
 17. A system as claimed in claim 1, including an intermediaterelaying station.
 18. A waterproof sonde comprising at least one sensorfor sensing at least one environmental parameter, a transmitter fortransmitting low frequency electromagnetic and/or magnetic signalscarrying data from the sensor to a control site using a magneticallycoupled antenna and a processor for assessing sensed environmentalparameters in relation to predetermined threshold levels.
 19. A sonde asclaimed in claim 18 comprising a receiver for receiving low frequencyelectromagnetic or magnetic signals from the control site using amagnetically coupled antenna.
 20. A sonde as claimed in claim 18 whereinthe sensor is operable to measure one or more of the following: flowrate; water depth by pressure sensing; water depth by ultrasonictechniques; temperature; acidity/alkalinity (pH); conductivity;dissolved oxygen content; chlorophyll density; salinity and presence ofchemicals such as ammonium salts and nitrates; presence of otherspecific pollutants determined by chemical analysis sensors; presence ofblue-green and other potentially toxic algae; particulate content; andturbidity.